1. Field of the Invention
The present invention relates to a method of checking a wirebond condition. In particular, the present invention relates to a method of checking a wirebond condition that is advantageously used for determining whether or not a metal wire is properly fixed to an object such as a semiconductor chip or a lead terminal.
2. Description of the Related Art
An example of the conventionally well-known wire-bonding techniques for electrically connecting two elements (such as a semiconductor chip and a lead) is a ball-bonding method. A typical ball-bonding operation using a bonding tool called xe2x80x9ccapillaryxe2x80x9d may be performed in the following manner.
First, a lower tip of a metal wire extending vertically through a bonding tool is melted into a ball. Then, the bonding tool is lowered toward a semiconductor chip to bring the ball-like end of the wire into contact with the semiconductor chip. Under a predetermined squeezing force, the ball-like end of the wire will be deformed between the bonding tool and the semiconductor chip. In this state, ultrasonic vibrations are applied to the bonding tool for a certain period of time. As a result, the lower end of the wire will be fixed to the semiconductor chip.
Though having many advantages, the conventional wire-bonding technique is disadvantageous in the following points.
It is now assumed that the semiconductor chip is mounted on a center pad (called xe2x80x9cislandxe2x80x9d) of a metal lead frame which in turn is carried by a flat supporting base. In such an instance, if the lead frame is not flat but warped, the central pad together with the semiconductor chip may be raised above the supporting base. When a wire-bonding operation is performed to such a raised semiconductor chip, the wire may fail to be firmly attached to the semiconductor chip.
Conventionally, the checking of wirebond condition is performed after a wire-bonding operation (that is, the wirebond condition checking is additionally performed separately from the wire-bonding operation). In this manner, however, the overall process for producing semiconductor devices tends to become time-consuming. Further, in practice, all of the semiconductor devices in the making are not subjected to the checking of wirebond condition (i.e., only arbitrarily selected ones are examined). Thus, conventionally, a certain number of defective semiconductor devices may disadvantageously be subjected to a second wire-bonding operation (or any other subsequent procedure) together with non-defective semiconductor devices.
It is, therefore, an object of the present invention is to provide a method of checking a wirebond condition which eliminates or reduces the above-described problems.
According to a first aspect of the present invention, there is provided a method of checking a wirebond condition produced by bonding a conductive wire to an object, the wire being guided by a bonding tool, the method comprising the steps of:
detecting a first position of the bonding tool when the bonding of the wire is completed;
applying a pulling force to the wire, the pulling force being small enough to prevent breakage of the wire;
detecting a second position of the bonding tool while the pulling force is being applied to the wire; and
comparing the first and the second positions of the bonding tool with each other.
With such an arrangement, it is possible to know whether the bonding of the wire to the object is properly performed or not simply by comparing the first and the second positions of the bonding tool. Thus, there is no need to perform the checking of the wirebond condition separately from the wire-bonding operation.
Preferably, the above method further comprises the step of sounding an alarm when the first and the second positions of the bonding tool differ from each other.
Instead of or in addition to the above alarm-sounding step, the checking method may comprise the step of canceling a subsequent bonding operation when the first and the second positions of the bonding tool differ from each other.
According to a second aspect of the present invention, there is provided a method of checking a wirebond condition produced by bonding a conductive wire to an object, the method comprising the steps of:
detecting a first position of the object when the wire is brought into contact with the object;
detecting a second position of the object when the bonding of the wire is completed;
calculating a difference between the first and the second positions of the object; and
comparing the calculated difference with a predetermined value.
With such an arrangement, it is possible to know whether the bonding of the wire is properly performed or not simply by comparing the calculated difference with a predetermined value. The predetermined value may be obtained by performing an experiment.
Preferably, the above method further comprises the step of sounding an alarm when the calculated difference is greater than the predetermined value.
Instead of or in addition to the above alarm-sounding step, the checking method may further comprise the step of canceling a subsequent bonding operation when the calculated difference is greater than the predetermined value.
In a preferred embodiment, the wire is guided by a bonding tool through which the wire extends.
The detecting of the first and the second positions of the object may be performed based on detection of positions of the bonding tool.
Other objects, features and advantages of the present invention will become clearer from the detailed description of preferred embodiments given below with reference to the accompanying drawings.